Electrical capacitors are electrical charge storage devices composed generally of a pair of conductors separated by a dielectric material. Capacitors may be used in both direct current (DC) and alternating current (AC) applications for a variety of purposes, including energy storage, signal coupling, motor starting, motor running, power factor correction, voltage regulation, VA efficiency, tuning, resonance, surge suppression, and filtration. In either AC or DC networks, capacitors may be arranged in series, shunt, and hybrid configurations to provide many operational advantages, both transient and steady state. For example, shunt capacitors can serve as current sources or voltage sources in both AC and DC applications and provide VAR support and power factor correction in AC applications.
In transient AC networks, capacitors can be used to improve power factor during transient conditions, which results in increased efficiency or other desirable enhancements. Transient applications of series capacitors include voltage surge protection, motor starting, current limiting, switching operations, and the like. For example, low power factor transient currents are associated with fault currents and inrush currents due to motor starting and transformer magnetization. Series capacitors can moderate these effects by improving overall power factor and network voltage regulation during the transient condition. In addition, series capacitance can provide a degree of current limiting during transient conditions as a result of the series impedance of the capacitor, thus reducing the magnitude of fault currents and, as a result, reducing generator, transformer, switchgear, bus and transmission line requirements. Further, mechanical stress associated with bringing additional generation capacity on line can be moderated by the presence of series capacitive coupling. While these and many other series capacitor advantages are well known, unit cost, size requirements, voltage limitations, current limitations, dv/dt limitations, di/dt limitations, insulation limitations, dielectric limitations, electromechanical limitations and thermodynamic limitations, have prevented widespread implementation of series capacitors, especially in low frequency applications.
Steady state AC network characteristics also can be improved through the incorporation of capacitors. For example, high capacitance, series applications impress a low steady state AC voltage on the capacitor, which can be beneficial when electrical transfer devices are used in conjunction with series capacitor banks. Similarly, electrical wave distortion can be reduced by altering capacitance. Certain electrical circuit parameters are optimized through impedance matching or detuning of series capacitors. Other circuits can be enhanced by the use of capacitors to provide current limiting and/or voltage division. Steady state series capacitor applications include motor running, filtration, power factor correction, efficient power transfer, voltage boosting, and the like. Series, shunt and hybrid capacitor arrangements can be employed to enhance motor torque, speed, efficiency, power, power factor, VA efficiency, coupling and the like. Various capacitor bank and motor winding configurations can also allow induction generators to power induction motors by providing the required magnetizing currents for both devices. In such an application, power quality can be improved, while reducing the cost of electric grid alternative sources, emergency power supplies, mobile equipment, and portable generators. Further, operational variation of capacitance and capacitive reactance can be used to enhance electrical network steady state performance.
The characteristics of DC networks also can be improved through the use of capacitors. In DC networks, capacitors can be used to moderate rapid changes in DC network voltage, to store energy for sudden increases in demand, and to absorb energy when the DC network is subjected to sudden increases in source current or decreases in load current. Capacitors are used to block DC. They are further employed to couple signals in predominantly DC applications and in resonant DC links. However, low ratios of instantaneous and steady state power capability to total stored energy tend to limit the operating utility of capacitors in DC applications. High ESR and overheating often limit the utility of conventional capacitor selections such as electrolytic capacitance in DC and signal coupling applications.
Capacitors typically are categorized as either non-polar or polar; and there are many realizations of each category. Non-polarized capacitors generally are useful in both DC and AC applications. Unfortunately, non-polarized capacitors-especially in series configurations-are not well-suited for many AC and DC applications due to limitations in size, capacitance, weight, efficiency, energy density, and cost. Singular polarized capacitors traditionally have been limited to use in DC and small AC signal coupling applications due to their unidirectional, forward biasing requirements. In addition, anti-series polarized capacitors can be used in transient applications, such as motor starting, and forwardly biased anti-series polarized capacitors can be continuously operated in AC applications. In DC applications, polarized capacitors are widely used for filtering, such as in the output stage of DC power supplies. Polarized capacitors are also used to couple signals between amplifier stages. Finally, polarized capacitors have historically been used as rectifiers.
Non-polarized capacitors commonly are constructed of two conductors separated by a dielectric or insulator. The conductors typically are made of a conductive material, such as copper, aluminum, other metal, or doped semiconductor. The dielectric or insulator may be composed of air, mica, oil, paper, plastic or other compound. Non-polarized capacitors also may be constructed as metalized film capacitors which are composed of a thin layer of plastic having metalized surfaces. The capacitance of non-polarized capacitors generally is limited by the surface area of the discrete conductors, the distance separating the conductors, and the dielectric constant. The rated voltage of such capacitors is limited by the dielectric constant, dielectric strength, and material and fabrication defects. The current and rate of change of current (i.e., di/dt) is limited by the, ESR, mechanical strength and thermodynamic properties of the particular capacitor materials and structure. Metalized film capacitors routinely short at points of minimum dielectric thickness. The subsequent burn through or fault clearing is sometimes referred to as self healing. Perhaps progressive self destruction would be a more accurate description of this behavior. The failure mechanism of shorting and then burn through can be disruptive in sensitive circuits such as digital devices. Further, metalized film capacitors tend to poorly dissipate heat. This creates internal hot spots and tends to accelerate capacitor failure.
Parallel-plate-type capacitors generally constitute the most common commercial realizations of the non-polarized capacitor. In such implementations, dielectric breakdown and failure of such capacitor embodiments often are associated with concentrations of charge accumulations at corners and sharp points of the conductive plates and material defects and variation of thickness in high electric field conditions. Although the capacitor can be designed and the dielectric material chosen such that the capacitor theoretically should withstand such conditions, conventional macroscopic manufacturing methods often do not provide the accuracy and control needed to ensure that the fabricated capacitor can perform at its theoretical capability. For example, conventional techniques cannot ensure that sharp corners or burrs on the conductors will be avoided, or that the thickness of the dielectric material will be uniform throughout its area, or that the dielectric will be disposed on the conductors in a conformal manner. Further the surface area of parallel-plate-type capacitors has been generally limited to flat place construction and conventional enhancement techniques such as plate sharing and spiral wound packaging.
Polarized capacitors have enhanced surface area as compared to non-polarized capacitors, which, unfortunately, introduces additional capacitor components, a charge transport mechanism, and additional losses. For example, the physical composition of one commonly used polarized capacitor—an electrolytic capacitor—includes a conductor, anode foil, anodized layers, liquid impregnated paper layer, insulation paper layer, cathode, and conductor. The construction methods and loss mechanisms for other polarized devices. (symmetric and asymmetric) such as super capacitors, ultra capacitors and double layer capacitors are similarly well known. However, polarized capacitors (as well as other polarized electric charge storage (PECS) devices), generally have a low cost per unit of capacitance and smaller mass and dimensions as compared with their non-polarized counterparts. These characteristics favor the use of polarized capacitors over non-polarized capacitors.
Despite these advantageous properties, polarized capacitors also have their drawbacks. The electrically directional capacitance versus rectification circuit behavior due to electron tunneling is often disadvantageous. As another example, polarized capacitors exhibit a higher equivalent series resistance (ESR) at power frequencies than the non-polarized type due to the resistance of the paper/electrolyte and power losses in the oxide (i.e., dielectric) layer. Further, electrolytic capacitors outgas hydrogen due to the electrolysis of water, and ion transport limitations and conductor termination practices tend to contribute to a steep frequency response curve. Still further, the maximum AC ripple current that can be tolerated by electrolytic capacitors is limited by the ESR, rated voltage and the thermodynamic, mechanical, and venting properties of the capacitor package that allow it withstand the resultant heat and pressure buildup without rupturing. Further, the most commonly used material, aluminum, requires great energy to refine conventionally. The anode etching and forming process then requires additional large inputs of energy, chemical processing and handling. Other conventionally constructed polarized charge storage devices suffer innumerable similar disadvantages.
Certain known methods exist for improving the thermodynamic properties of polarized capacitors. These methods include increasing thermal mass by increasing foil thickness, increasing fluid volume and the use of thicker can material. It is also possible to increase heat dissipation by reducing the thermal resistance to heat flow. This is accomplished by such methods as crimping the cathode foil to the can, increasing the surface area of the can internally and externally and creating additional thermal structures such as cold fingers, headers and stud mounting. Another known methods include increased air flow, circulating fluid and other external heat control methods. Finally increased radiation and conduction can be achieved by means of increasing the capacitor allowable operating temperature. These methods, though somewhat effective tend to increase costs substantially and in many cases substantially increase the physical size and weight of the components.
Typically, for both polarized and non-polarized discrete capacitors, neither the theoretical dielectric strength nor the theoretical dielectric constant, have been effectively realized due to material imperfections, imprecise manufacturing processes, and boundary interface problems. These factors, in turn, limit both the maximum rated device voltage and capacitance that may be attained for a given capacitor implementation. Still further, imbalances in conduction current and displacement current capabilities combined with inconsistent material properties limit the transient and sustained current capabilities for a given capacitor. Structural thermodynamic limitations further tend to limit transient and steady state electrical current capabilities and capacitor operational lifetime. Accordingly, there is a need to provide improved capacitors and methods for fabricating capacitors that result in increased capacitance, voltage and current ratings, and power delivery.
It is well known that capacitance in flat plate capacitors is governed by the following equation:C=E0ERA/d where E0 is the permittivity of free space, ER is the relative permittivity of the dielectric, A is the common surface area of the conductors, and d represents the distance between conductors. From the foregoing equation, it can be seen that capacitance can be increased by increasing the common surface area A of the conductors. FIG. 1 shows an instantaneous charge accumulation on the conductor plates 10 and 11 of a generalized capacitor 15 having a planar surface for the conductive layers. Microscopic charge displacement in the dielectric allows current flow. Positive and negative charges are shown. A dielectric layer 13 is disposed between the conductor plates 10 and 11.
An example of a known technique for increasing surface area can be seen in FIG. 2, which represents a magnified cross-sectional view of an exemplary embodiment of a polarized electrolytic capacitor 20 having conductor foils 22 and 24. The surface area of the foils 22 and 24 is increased by acid etching the conductors such that microchannels 26 are formed. The microchannels 26 typically are on the order of 40 μm by 1 μm and have sharp edges. The high purity aluminum anode 22 is oxidized by known large scale fabrication methods to create a thin film of aluminum oxide in either crystalline, polycrystalline or amorphous form to create a dielectric layer 28 having a relative dielectric constant ER of approximately 9. The insulation rating, corresponding to such a dielectric constant is generally; on the order of 1.1 nM/V.
It can be seen from FIG. 2 that the effective surface area of the conductor foils is increased substantially as a result of the broom-straw-like structure. However, it is difficult to charge the capacitor, particularly at high voltages due to spatial distance variations between the extremities of the broom-straw-like structures and the attendant displacement current limitations. To remedy this inherent weakness, an additional charge transport mechanism is introduced in the form of a paper wet with an electrolytic solution to provide a pathway for electrical charges to reach the enhanced surface area of the conductor during the charging process.
The configuration illustrated in FIG. 2 has many characteristics which ultimately limit the performance and longevity of the capacitor. For example, negative ions, which travel from the cathode foil to the anode foil through the wetted paper during the charging process, increase the ESR of the capacitor and limit ripple current ratings. Hydrogen gas emitted during the charging process due to the electrolysis of water must be vented. Mechanical weakness of the structure and required anodization thickness limit capacitor rated voltage. And, although the microchannels serve to increase the surface area of the conductors, the effect of this enhancement is reduced from two orders of magnitude to one order of magnitude as rated voltages are increased.
Another drawback to aluminum electrolytic capacitors is the enormous quantity of energy required for fabrication. Aluminum has been referred to as congealed electricity. The energy required for high purity aluminum, such as required for anodic foil is greater still. Conventional manufacturing typically requires processing with first strong alkaline and then strong acid chemical baths in an impressed electrical field. Several washes of high purity water are also required. Great amounts of electrical power are required for heating, oxidizing and forming the aluminum foil and tab materials. The electrolyte solution is often a petrochemical such as ethylene glycol mixed with water and other chemicals such as acids or bases. Winding, wetting and stuffing operations are followed by final electrical formation steps. These steps and inputs are highly energy intensive. Thus, conventional manufacturing techniques for aluminum electrolytic capacitors require a substantial quantity of energy.
Anti-series pairs of polarized capacitors suffer from several disadvantages. First if the pair is unbiased, one device acts as a capacitor while the other component acts as a diode. This operating condition alternates every half cycle and greatly shortens capacitor assembly life and is a source of electrical harmonic current and ground reference voltage disturbances. When equal size, anti-series capacitors are biased, the capacitance of the assembly is cut approximately in half. ESR and related high dissipation factor are increased for the assembly, as they are series additive electrical phenomena.
Small-scale manufacturing techniques also are known for fabricating capacitors. For example, semiconductor manufacturing techniques are used to create capacitors in solid state integrated circuit devices. Because an object of integrated circuit memory designs is to create short half life circuits at low voltages, such designs focus on reducing capacitance often and favor lower dielectric constants rather than increasing capacitance and enhancing power delivery characteristics. Where high dielectric constants and current density have been favored in these applications the purpose is generally in pursuit of miniaturization and ever lower capacitance. Decoupling capacitors act as localized, low impedance voltage sources; thus furnishing noise free power to synchronous integrated circuits. Printed circuit board electrical, thermal and mechanical limitations severely limit integrated capacitor materials and construction techniques. Also integrated capacitance variation cannot be easily controlled using conventional manufacturing techniques.
Other polarized electrical charge storage device research has revolved around increasing total energy storage and has resulted in the development of super capacitors, ultra capacitors or double layer capacitors. Such capacitors are intended to bridge the gap between electrochemical batteries and polarized capacitors, such as liquid tantalum and aluminum electrolytic capacitors. Energy storage capability is increased in super, ultra and double layer capacitors by enhancing conductor surface area and volume charge storage capabilities by large-scale manufacturing techniques such as those described in U.S. Pat. No. 5,876,787, entitled “Process of Manufacturing a Porous Carbon Material and Capacitor having the Same.”
Super capacitors, ultra capacitors and double layer capacitors, however, have many limiting characteristics which inhibit their usefulness for power applications. For example, such capacitors have relatively low voltage ratings (i.e., 1V-3V per cell) and tend to have relatively high ESR, both of which are not positive attributes in applications having power transfer as an object. Further, the devices are polarized charge storage devices, thus restricting their usefulness in AC power applications. Further, such devices often fail to deliver the full charge stored on demand. A great deal of the stored charge can remain unavailable. This observed characteristic has a time dependant component and a time invariant component. Not all the stored energy which can be put to use, can be released instantaneously, making the devices less suitable for rapid rate charge and discharge applications. The second mechanism by which the stored charge remains unavailable for convenient use is the phenomenon of trapped energy. Series assemblies comprised of capacitors of various sizes and charge levels will retain a significant and measurable voltage trapped within, at the end of discharge. The low cell voltages of super, ultra and double layer capacitors require many cells to achieve common system voltages. This phenomenon can also be observed in electrochemical battery discharges and is sometimes referred to as cell inversion.
Improvements in power delivery and end use systems can have a significant impact on today's economy and environment. More particularly, electrical motors presently consume about 65% of metered real power. To illustrate the improvements that can be realized, assume that an example motor has a 50% power factor and that the remaining 35% of metered load is purely resistive. Thus, the total Volt-Amps (VA) of the combined load is 119.27% of the real power, and the 35% resistive load is only 29.24% of the total VA load. Accordingly, the motor load in this example is greater than 70.75% of the system total VA load. Capacitors arranged in series, shunt, and hybrid configurations can help economically to correct motor power factor and reduce the economic and environmental consequences associated therewith. Further, certain LC motor designs have been demonstrated to provide increased motor efficiency, torque, power factor, vibration, phase-leg-loss and other desirable motor properties over purely magnetic designs thus also improving economics and the environment.
Such improvements in power delivery and end use systems and the accompanying benefits can be realized by an enhanced discrete non-polarized capacitor having increased capacitance, heat dissipation and power transfer capabilities. Such improvements also could be realized by an enhanced discrete polarized capacitor having increased capacitance, increased voltage and ripple current ratings, reduced ESR, and improved heat dissipation and power transfer characteristics. The improved discrete capacitor characteristics and methods can also be beneficially applied to integrated circuits, digital chips and other electrical devices.